The present invention generally relates to power transmission systems. More particularly, this invention relates to power split transmission systems adapted for automotive applications, in which an energy recovery capability is provided by the inclusion of one or more energy storage devices.
Power transmission systems typically found in common automotive applications utilize a mechanical transmission system entirely made up of solid components including shafts, gears, and clutches, which alone may be used to transmit power to the drive wheels of a vehicle, as in the case of “manual” transmissions. Mechanical transmission systems are also used in combination with hydraulic transmission systems that use a liquid under pressure to transmit power, as in the case of an “automatic” transmission that uses a torque converter as a hydrodynamic fluid coupling.
Developments in the automotive industry, including passenger, commercial, and off-road vehicles, are shaped by a strong demand to reduce fuel consumption. In addition, there is a trend toward higher top speed capabilities in heavy off-road vehicles, where faster driving speeds on roads are desired. In response, different types of continuously variable transmissions (CVT) have been developed and brought to the market. Among the CVT concepts, hybrid drives are of particular interest to vehicle manufactures. Hybrid drives are based on the utilization of brake energy for vehicle propulsion by storing brake energy in a battery, fly wheel, hydraulic accumulator, or other energy storage means. By recapturing energy otherwise lost as heat during braking, the hybrid drive technology is capable of significantly reducing fuel consumption, particularly in heavy trucks and cars. Comprehensive overviews of hydraulic hybrid drives are provided in Stecki et al., “Advances in Automotive Hydraulic Hybrid Drives,” Proceedings of the Sixth International Conference on Fluid Power Transmission and Control, Hangzhou, China (2005), and Miller, “Comparative Assessment of Hybrid Vehicle Power Split Transmissions,” 4th VI Winter Workshop Series (2005). While CVT's provide seamless shifting in vehicle operation, allowing the engine to operate at a nominal speed range resulting in lower fuel consumption and emissions, typical CVT's suffer from either low shaft-to-shaft efficiency or low torque handling capabilities.
Power split transmissions (PST), also known as power split drive (PSD) transmissions, are a particular type of CVT that has found use in applications such as agricultural tractors, for example, the Fendt Vario line of tractors brought to the market by Fendt (AGCO Corporation) in 1996. See, for example, Dziuba et al., “Entwicklung eines neuen stufenlosen Schleppergetriebes mit hydrostatisch mechanischer Leistungsverzweigung,” VDI-Berichte Nr. 1393, VDI-Verlag, Düsseldorf, Germany (1998), p. 541-549 (in German). Although the principle of power split drives has been known for more than four decades, this technology is still in a developmental stage.
As known, PSD transmissions traditionally use a planetary (epicyclic) gear train (PGT) in combination with a continuously variable transmission that achieves continuous variable speed control along with high efficiency levels that are derived from the mechanical gears of the PGT. There are three basic implementations of power split drives: the combination of a PGT with a continuous variable mechanical gear; the combination of a PGT with a hydrodynamic transmission; and the combination of a PGT with a hydrostatic transmission. The last of these allows further fuel savings if a drive line control concept is implemented that takes engine characteristics into account. The engine speed can be adjusted to a point where the total power loss of the transmission is minimized, as reported in Ossyra et al., “Drive Line Control for Off-Road Vehicles Helps to Save Fuel,” SAE International Commercial Vehicle Engineering Congress, Chicago, Ill., USA, SAE Technical Paper 2004-01-2673 (2004). Through the improvement of the efficiency of positive displacement machines, the use of hydrostatic transmissions in PSD's has become very attractive for many different applications.
PSD transmissions have three different operating modes that are known in the automotive transmission industry under the following designations: power additive, full mechanical, and power recirculation. The power flows of these three modes are schematically represented in FIG. 1. In the power additive mode, the power, Pin, from a combustion engine (or other suitable power source) is split and transferred into two paths: a mechanical path (containing a planetary gear train), Pmech, and a hydraulic path (such as a hydrostatic transmission), Phyd. The power is then combined and transferred as Pout to the wheels to propel a vehicle. In full mechanical mode, power is entirely transferred from the engine to the wheels via the mechanical path (Pmech) and not through the hydraulic path (Phyd). Generally full mechanical mode is at a single speed or has a small speed range. In the power recirculation mode, some of the power transferred via the mechanical path (Pmech) is recirculated back through the hydraulic path (Phyd). The recirculated hydraulic power (Phyd) is combined with the engine power (Pin) from the engine and again transferred via the mechanical path (Pmech), thus being recirculated. In general, full mechanical mode is considered to be the most efficient transmission power mode for a PSD transmission, and the power recirculation mode is considered to be the least efficient transmission power mode because large amounts of power can be recirculated through the hydraulic path.
FIG. 2 identifies PSD transmissions categorized by families based on structural similarities—first, whether the hydraulic path (Phyd) is coupled to the input (input coupled) or to the output (output coupled) of the mechanical transmission system, and then further categorized by the characteristics of the planetary gear train (basic, multistage, or compound). A comparison of achievable efficiencies and operating characteristics of these structural approaches has been presented in Carl et al., “Comparison of Operational Characteristics in Power Split Continuously Variable Transmissions,” SAE Commercial Vehicle Engineering Congress and Exhibition, Chicago, USA, SAE Technical Paper 2006-01-3468 (2006).
The basic output and input-coupled types are represented in FIGS. 3 and 8, respectively. FIG. 3 shows a basic output-coupled PSD transmission 10 as utilizing a hydrostatic transmission 12 as the hydraulic path and continuously variable part of the transmission 10. The hydrostatic transmission 12 is mechanically coupled (via a gear set) to the output of a simple planetary gear train 14 (i.e., not multistage or compound), which serves as the mechanical transmission system (path) of the transmission 10. The planetary gear train 14 is mechanically coupled (via a shaft) to a combustion engine 11 as the power source of the vehicle in which the transmission 10 is installed. The outputs of the hydrostatic transmission 12 and planetary gear train 14 are both mechanically coupled (via a gear set and a shaft, respectively) to the drive axle and wheels 19 of the vehicle. The hydrostatic transmission 12 comprises two positive displacement units 16 and 18, labeled “Unit 1” and “Unit 2” in FIG. 3. As understood in the art, the positive displacement units 16 and 18 operate by trapping and then displacing a fixed volume of hydraulic fluid. As such, the speed of the vehicle can be controlled by controlling the displacements of the units 16 and 18 using the vehicle velocity as a feedback signal. The output-coupled transmission 10 of FIG. 3 is limited to two operational modes: power additive and power recirculation. The output-coupled transmission 10 operates in the power additive mode at low speeds and the power recirculation mode at high speeds. With constant engine speed, increasing the vehicle forward velocity from standstill is achieved by increasing the displacement of the unit 16 from zero to maximum, then decreasing the displacement of the unit 18 from maximum to zero. Reverse is achieved by running the unit 16 over center. The differential pressure in the hydrostatic transmission 12 is determined by the load torque. During braking, the high pressure and low pressure lines switch as the units 16 and 18 change pumping and motoring modes.
In FIG. 8, a basic input-coupled PSD transmission 50 is shown as utilizing a hydrostatic transmission 52 as the hydraulic path and variable part of the transmission 50, and a planetary gear train 54 that serves as the mechanical path of the transmission 50. In contrast to the output-coupled PSD transmission 10 of FIG. 3, the hydrostatic transmission 52 and the planetary gear train 54 are both mechanically coupled (via a gear set and a shaft, respectively) to a combustion engine 51, the hydrostatic transmission 52 is mechanically coupled (via a gear set) to the input of the planetary gear train 54, and the output of only the planetary gear train 54 is mechanically coupled (via a shaft) to the drive axle and wheels 59 of the vehicle. Similar to the transmission 10 of FIG. 3, the speed of the vehicle is controlled by controlling the displacements of two positive displacement units 56 and 58 of the hydrostatic transmission 52, labeled “Unit I” and “Unit II” in FIG. 8, using the vehicle speed as feedback.
As with the output-coupled transmission 10 of FIG. 3, the input-coupled transmission 50 is limited to two operational modes: power additive and power recirculation. The transmission 50 operates in power recirculation mode at low speeds, and at higher speeds operates in power additive mode. This cycle can be repeated several times by adding clutches and more advanced planetary gear trains (e.g., multistage and compound). With constant engine speed, increasing the vehicle forward velocity from standstill is achieved by increasing the displacement of the unit 56 from negative maximum through zero to positive maximum, then decreasing the displacement of the unit 58. Reverse is achieved by holding the displacement of the unit 56 at maximum and decreasing the displacement of the unit 58. The differential pressure within the hydrostatic transmission system 52 is simply a reactionary item, a function of the load torque on the wheels 59. Deceleration is only possible with standard friction-type brakes connected to the wheel axle.
In view of the above, both the output-coupled and input-coupled PSD transmissions have certain limitations and inefficiencies, such that additional developments and improvements would be desirable to further expand the technical and commercial viability of PSD transmissions.